This project is being submitted in two separate grant proposals to facilitate administration of funds. See accompanying proposal with the same title Robert B. Armstrong, Assistant Professor of Biology and Health Sciences, Boston University, Boston, MA. This proposal has two main objectives: a) to determine the manner in which muscles and populations of fibers within muscles are recruited during locomotion: 1) as animals increase speed and change gaits; and 2) as animals use their muscles for different tasks, i.e., positive mechanical work and elastic storage of energy. b) to investigate the contribution of skeletal muscle to total resting metabolism. We will use the following techniques: EMG's synchronized with high speed movies to determine the activity and length changes of active muscles; glycogen depletion as a technique to determine the cross-sectional area of muscles, populations of active fibers, and the power of active muscles; length changes of tendons (measured using high speed X-ray movies) as a means of measuring force exerted by a muscle during locomotion; and blood flow to muscle groups together with oxygen extraction from the blood to measure their oxygen consumption.